Virginie Thierry

Quality Control of Large Argo Datasets

Abstract Argo floats have significantly improved the observation of the global ocean interior, but as the size of the database increases, so does the need for efficient tools to perform reliable quality control. It is shown here how the classical method of optimal analysis can be used to validate very large datasets before operational or scientific use. The analysis system employed is the one implemented at the Coriolis data center to produce the weekly fields of temperature and salinity, and the key data are the analysis residuals. The impacts of the various sensor errors are evaluated and twin experiments are performed to measure the system capacity in identifying these errors. It appears that for a typical data distribution, the analysis residuals extract 2/3 of the sensor error after a single analysis. The method has been applied on the full Argo Atlantic real-time dataset for the 2000–04 period (482 floats) and 15% of the floats were detected as having salinity drifts or offset. A second test was per...

The Origin of Deep Zonal Flows in the Brazil Basin

Recent data from a deployment of Lagrangian floats in the Brazil Basin of the South Atlantic reveal a swift western boundary current and predominantly zonal flow in the interior at a depth of about 2500 m. Dynamical mechanisms for the deep interior flow are considered using two high-resolution models, a global and a regional one, together with a suite of sensitivity studies at low resolution. Outside the western boundary region, model energy levels are similar to observations. The models are able to reproduce, at somewhat reduced strength depending on resolution, much of the meridional structure of the observed deep zonal flows. Several candidates for generating such flows are examined, including nonlinear rectification, baroclinic instability, and thermohaline and wind forcing. A primary mechanism for the deep flow in the models is the response to the wind stress, as recently argued to be the case for a model of the Pacific Ocean. However, thermohaline forcing is significant, especially where density contrasts between basins generate strong currents in deep passages. The deep thermohaline flow appears to be linked to the depth of the midocean ridge. Baroclinic instability of the mean meridional flow, which is alone capable of generating nearly zonal currents of the observed scale, is a possible additional forcing but is not essential in the models investigated here. The meridional scale of the zonal flows in the models is extremely dependent on the horizontal resolution and horizontal mixing.

Overturning in the Subpolar North Atlantic Program: A New International Ocean Observing System

A new ocean observing system has been launched in the North Atlantic in order to understand the linkage between the meridional overturning circulation and deep water formation. For decades oceanographers have understood the Atlantic Meridional Overturning Circulation (AMOC) to be primarily driven by changes in the production of deep water formation in the subpolar and subarctic North Atlantic. Indeed, current IPCC projections of an AMOC slowdown in the 21st century based on climate models are attributed to the inhibition of deep convection in the North Atlantic. However, observational evidence for this linkage has been elusive: there has been no clear demonstration of AMOC variability in response to changes in deep water formation. The motivation for understanding this linkage is compelling since the overturning circulation has been shown to sequester heat and anthropogenic carbon in the deep ocean. Furthermore, AMOC variability is expected to impact this sequestration as well as have consequences for regional and global climates through its effect on the poleward transport of warm water. Motivated by the need for a mechanistic understanding of the AMOC, an international community has assembled an observing system, Overturning in the Subpolar North Atlantic (OSNAP), to provide a continuous record of the trans-basin fluxes of heat, mass and freshwater and to link that record to convective activity and water mass transformation at high latitudes. OSNAP, in conjunction with the RAPID/MOCHA array at 26°N and other observational elements, will provide a comprehensive measure of the three-dimensional AMOC and an understanding of what drives its variability. The OSNAP observing system was fully deployed in the summer of 2014 and the first OSNAP data products are expected in the fall of 2017.

In Situ–Based Reanalysis of the Global Ocean Temperature and Salinity with ISAS: Variability of the Heat Content and Steric Height

AbstractThe In Situ Analysis System (ISAS) was developed to produce gridded fields of temperature and salinity that preserve as much as possible the time and space sampling capabilities of the Argo network of profiling floats. Since the first global reanalysis performed in 2009, the system has evolved, and a careful delayed-mode processing of the 2002–12 dataset has been carried out using version 6 of ISAS and updating the statistics to produce the ISAS13 analysis. This last version is now implemented as the operational analysis tool at the Coriolis data center. The robustness of the results with respect to the system evolution is explored through global quantities of climatological interest: the ocean heat content and the steric height. Estimates of errors consistent with the methodology are computed. This study shows that building reliable statistics on the fields is fundamental to improve the monthly estimates and to determine the absolute error bars. The new mean fields and variances deduced from the ...

Origin, formation and variability of the Subpolar Mode Water located over the Reykjanes Ridge

The origin and formation of the Subpolar Mode Water (SPMW) located over the Reykjanes Ridge in the North-Atlantic Ocean and the variability of its properties over the period 1966-2004 are investigated through the use of a global eddy-permitting (1/4 degrees) ocean/sea-ice model and a Lagrangian analysis tool. The SPMW is fed by subtropical and subpolar waters advected by the branches of the North-Atlantic Current. The SPMW acquires its properties when its source waters enter the winter mixed layer in the Iceland Basin. The SPMW temperature variability is mainly explained by variations of the relative contributions of the subtropical and subpolar water transports to the total transport. Compared to the 1966-2004 mean, lower (higher) subtropical water relative transport contribution leads to colder (warmer) SPMW in the early 1990s (in the late 1960s and late 1990s). The intensity of the winter convection in the Iceland basin also influences the SPMW temperature through the amount of relatively cold intermediate waters of subtropical origin integrated in the SPMW layer. Strong convection partly explains the cold SPMW of the early 1990s. The large increase in the SPMW temperature in the late 1990s is due to both a decrease in the winter convection and an increase in the relative transport of the subtropical waters.

Mixed layer heat budget in the Iceland Basin from Argo

The mixed layer processes that govern the mode water properties in the Iceland Basin are quantified through a mixed layer heat budget from Argo data collected over 2001-2007. This budget includes the mixed layer heat content variation, the surface heat fluxes, and the Ekman contribution to advection. The geostrophic advection cannot be directly estimated from Argo data but, following previous works, an ad hoc procedure is implemented to take it into account. The resulting annual budget is closed within the error bar but this closure hides some compensation between the summer and the winter residuals (-16 +/- 9 W m(-2) and 21 +/- 26 W m(-2), respectively). A similar heat budget built by using colocated Argo floats in the 1/4 degrees DRAKKAR ORCA025-G70fo simulation over 2001-2007 shows seasonal patterns similar to the Argo-based budget. An Eulerian model-based heat budget in the Iceland Basin shows that the mixed layer heat content variation is driven by the air-sea fluxes, the advection, and the vertical diffusion. The indirect estimate of the latter in a new Argo-based budget leads to a summer residual of 2 +/- 11 W m(-2) and to an unchanged winter residual. The summer and winter standard errors of the Argo-based budget (11 and 26 W m(-2), respectively) reflect the limited sampling of the Iceland Basin by Argo floats. Sensitivity experiments show that such errors would be reduced by a denser Argo sampling.

Simulated decadal variability of the meridional overturning circulation across the A25-Ovide section

andhighlatitudes(12Sv,1Sv=10 6 m 3 s –1 ) and a cellinternal to the subpolar gyre(4Sv). The decadal MOC variability is associated with synchronized transport changes of the subtropical and subpolar inflow within the North Atlantic Current (NAC). The varying strength of the MOC is further related to changes in the upper horizontal transport distribution. When the MOC is in a strong phase (early 1990s), the northern branch of the NAC in the Iceland Basin is strong while the southern branch at the Rockall Trough entrance is relatively weak. The inverse situation holds for a persistent weak MOC state (1970s). Contrary to the conclusions of earlier studies, variability in the strength and shape of the subpolar gyre does not stand as the main driver of the changing NAC structure, which is largely induced by the horizontal variability of the subtropical inflow. Additionally, the recently shown intrusion of subtropical waters into the northeastern Atlantic (late 1960s, early 1980s, and 2000s) are shown to primarily occur during periods of weak MOC circulation at A25-Ovide.

Gyre scale deep convection in the subpolar North-Atlantic Ocean during winter 2014–2015

Using Argo floats, we show that a major deep convective activity occurred simultaneously in the Labrador Sea (LAB), South of Cap Farewell (SCF) and the Irminger Sea (IRM) during winter 2014–2015. Convection was driven by exceptional heat loss to the atmosphere (up to 50% higher than the climatological mean). This is the first observation of deep convection over such a widespread area. Mixed layer depths exceptionally reached 1700 m in SCF and 1400 m in IRM. The deep thermocline density gradient limited the mixed layer deepening in the Labrador Sea to 1800 m. Potential densities of deep waters were similar in the three basins (27.73-27.74 kg m−3), but warmer by 0.3 °C and saltier by 0.04 in IRM than in LAB and SCF, meaning that each basin formed locally its own deep water. The cold anomaly that developed recently in the North-Atlantic Ocean favored and was enhanced by this exceptional convection.

Global and full-depth ocean temperature trends during the early 21st century from Argo and repeat hydrography

The early 21st century’s warming trend of the full-depth global ocean is calculated by combining the analysis of Argo (top 2000m) and repeat hydrography into a blended full-depth observing system. The surface-to-bottom temperature change over the last decade of sustained observation is equivalent to a heat uptake of 0.72 ± 0.09 W m?2 applied over the surface of the earth, 90% of it being found above 2000m depth. We decompose the temperature trend point-wise into changes in isopycnal depth (heave) and temperature changes along an isopycnal (spiciness) to describe the mechanisms controlling the variability. The heave component dominates the global heat content increase, with the largest trends found in the southern hemisphere’s extratropics (0 - 2000m) highlighting a volumetric increase of subtropical mode waters. Significant heave-related warming is also found in the deep North Atlantic and Southern Ocean (2000m - 4000m), reflecting a potential decrease in deep water mass renewal rates. The spiciness component shows its strongest contribution at intermediate levels (700m - 2000m), with striking localised warming signals in regions of intense vertical mixing (North Atlantic and Southern oceans). Finally, the agreement between the independent Argo and repeat hydrography temperature changes at 2000m provides an overall good confidence in the blended heat content evaluation on global and ocean scales, but also highlights basin scale discrepancies between the two independent estimates. Those mismatches are largest in those basins with the largest heave signature (Southern Ocean) and reflect both the temporal and spatial sparseness of the hydrography sampling.

Variability of the Turbulent Kinetic Energy Dissipation along the A25 Greenland–Portugal Transect Repeated from 2002 to 2012

AbstractThe variability of the turbulent kinetic energy dissipation due to internal waves is quantified using a finescale parameterization applied to the A25 Greenland–Portugal transect repeated every two years from 2002 to 2012. The internal wave velocity shear and strain are estimated for each cruise at 91 stations from full depth vertical profiles of density and velocity. The 2002–12 averaged dissipation rate 〈e2002–2012〉 in the upper ocean lays in the range 1–10 × 10−10 W kg−1. At depth, 〈e2002–2012〉 is smaller than 1 × 10−10 W kg−1 except over rough topography found at the continental slopes, the Reykjanes Ridge, and in a region delimited by the Azores–Biscay Rise and Eriador Seamount. There, the vertical energy flux of internal waves is preferentially oriented toward the surface and 〈e2002–2012〉 is in the range 1–20 × 10−10 W kg−1. The interannual variability in the dissipation rates is remarkably small over the whole transect. A few strong dissipation rate events exceeding the uncertainty of the fi...